1. Field of the Invention
The invention relates in general to a probe, and more particularly, to a flexible probe used to measure microwave and high-speed signals from an uneven surface.
2. Description of the Related Art
In addition to the printed circuit board (PCB), the versatile applications of microwave and high-speed signal circuit further include the hybrid circuit, the multi-chip module (MCM), and the integrated circuit (IC). For the microwave circuit and the high-speed signal circuit, the most accurate and convenient measurement methods include on-wafer measurement. To obtain an accurate measurement result, a good probe is required.
The technique of probe used for on-wafer measurement has been disclosed in U.S. Pat. No. 4,697,143 in September 1989. This invention of “Wafer Probe” issued to Larry R. Lockwood et al. is a probe made with Al2O3 substrate for signal measurement. The measured signal from the tip is transmitted through the transmission line on the substrate to a planar-coaxial cable converter, and is then output to a measurement instrument. In U.S. Pat. No. 4,697,143, the microwave probe has a planar probing structure that is fabricated on the hard Al2O3 substrate for signal transmission. Such structure is used to probe the object to be measured. In addition to a probe tip, the signal is transmitted through the probe tip, along a signal transmission planar structure on the Al2O3 substrate to the coaxial transmission structure. The signal transmission medium made of Al2O3 has the characteristics of high microwave transmission quality and low transmission loss. However, as the supporting substrate is made of a hard material, the contact angle between the probe and the object to be measured needs to be fixed and the downward pressure applied to the probe is strictly specified during measurement. Therefore, the probe tip has to be held parallel to the object to be measured. Further, only a limited range of pressure can be applied to the probe while touching the object, for avoiding damaging the probe.
Later in April 1996, Edward M. Godshalk disclosed “high-frequency probe tip assembly” in the U.S. Pat. No. 5,506,515. One end of the planar transmission structure, which is void of supporting substrate and is in a claw shape, is used as the probe tip, so as to allow measuring a non-planar object. A planar-coaxial converter is adjacent right behind the probe to output the signal to the measurement instrument thereby. In U.S. Pat. No. 5,506,515, the claw-shaped probe tip can be shifted up and down within a limited range for measuring an object with an uneven or non-planar surface. The problems relating to the contact angle and the downward pressure are thus partially resolved. However, because only the claw-shaped probe tip has flexibility, the tolerance of the contact angle for probing the object is still limited by the length and pitch of the probe tip. That is, the longer the probe tip is and the smaller the pitch is, the bigger is the tolerance of the angle.
Further in June 1996, Markku Jenu disclosed “rugged probe design MIC measurement” in the Microwave & RF (Radio Frequency) magazine. A soft substrate is incorporated as the supporting material for the probe tip and the signal transmission planar structure. Such substrate allows the probe tip having angle differences (for example, angle of depression or angle of elevation) with the object to be measured. In this disclosure, different material layers (with different dielectric constants) are laminated into a composite substrate, of which one layer with a higher dielectric constant is harder and used as the dielectric material for signal transmission planar structure, and another layer with a lower dielectric constant is softer and provides supports and flexibility. However, the probe tip and the signal transmission planar structure are all disposed on the composite substrate. As the whole structure of the probe is resilient, the angle tolerance of the probe tip for measuring the object is increased. However, being formed on the composite substrate, the probe tip moves along with the composite substrate, rather than moving up and down separately. Thus, this kind of probe is not applicable for measuring the object with an uneven surface.